MR FLOW MEASUREMENT USING RF PHASE GRADIENTS IN RECEIVER COIL ARRAYS

Radio frequency (RF) phase gradients in the receiver coil field patter n can encode flow velocity information in magnetic resonance (MR) imag es in the form of phase variations. These phase variations are not rea dily observed in MR images because they are relatively small compared to phase variations caused by static magnetic field (B0) inhomogeneiti es, susceptibility variations, and other sources. However, the phase c ontributions from these other sources are independent of the receiver coil. Therefore, the RF phase gradient encoded flow information can be recovered by subtracting images obtained simultaneously using arrays of independent receiver coils and a multiple channel receiver. This fl ow velocity information can be extracted retrospectively from standard imaging sequences, including flow-compensated sequences. No additiona l time is required for the flow study as the flow measurements are mad e using sequences chosen for optimal imaging, and the images from each coil are obtained simultaneously. Initial results indicate that suffi cient sensitivity is obtained to make flow measurements in the range o f velocities commonly found in the carotid arteries and other major ve ssels. In principle, the method works with only two receiver coils. Ho wever, additional elements provide additional phase measurements that can be used to increase accuracy, remove ambiguities in flow direction or velocity calculations, and increase the region over which velocity measurements can be accurately made.